1. Field of the Invention
The present invention relates to a sheet feeding apparatus suitable to be connected to an image forming system such as a copying machine, printer, facsimile and the like, and more particularly, it relates to an apparatus for positioning a recording sheet on which an image is to be formed in a transverse or lateral direction.
2. Related Background Art
In an image forming system such as a copying machine, printer, facsimile and the like (referred to as "copying machine and the like" hereinafter), in order to form an image on a recording sheet at a correct position, the recording sheet must be supplied to the image forming system with proper alignment of the recording sheet. The correction of the alignment of the recording sheet, i.e., the correction of tile skew-feed of the recording sheet is generally performed in a sheet supplying/feeding apparatus. In this case, the correction methods are generally grouped into two kinds depending upon which portion of the recording sheet is used as a reference.
A first correction method utilizes a sheet supplying/feeding apparatus wherein a leading end of a recording sheet is used as a reference. An example of such apparatus is shown in FIG. 9.
Such apparatus comprises sheet feed rollers 91 for feeding the recording sheet P in a direction shown by the arrow A, and a pair of upper and lower registering rollers 92a, 92b disposed at a downstream side of the sheet feed rollers 91. When the recording sheet P continues to be fed after a leading end P1 of the recording sheet P has been abutted against a nip 93 between the registering rollers 92a, 92b now stopped, a loop is formed in a portion of the recording sheet P between the sheet feed rollers 91 and the nip 93 to correct the skew-feed of the sheet. By forming the loop, the leading end P1 of the recording sheet P is urged against the nip 93, thus positioning the leading end P1 of the sheet along the nip 93 correctly. Thereafter, when the registering rollers 92a, 92b are rotated, the recording sheet P is fed in a condition that the skew-feed of the sheet is corrected by using the leading end P1 thereof as the reference. In this case, the lengths of the registering rollers 92a, 92b in a left-and-right direction (transverse direction) are so selected that a lateral width of the nip 93 becomes greater than a length of the leading end P1 of the recording sheet P.
A second method for correcting the skew-feed of the recording sheet P utilizes one lateral edge P2 of a recording sheet P as a reference, as shown in FIG. 10.
According to this second method, a reference guide 95 is disposed along a recording sheet feeding path, and, by a sheet feed roller (not shown) and skew-feed rollers 97, the recording sheet P is fed forwardly (in a direction A) and at the same time is shifted to a transverse direction (shown by the arrow B). The skew-feed rollers 97 are inclined at predetermined skew-feed angles .alpha.1, .alpha.2, respectively, so that the recording sheet P being moved forwardly is shifted laterally by forces (referred to as "skew-feed forces" hereinafter) depending upon the skew-feed angles .alpha.1, .alpha.2, thus slidingly contacting the lateral edge P2 of the recording sheet with the reference guide 95. In this way, the skew-feed of the recording sheet is corrected by using the lateral edge P2 thereof as the reference.
FIG. 11 shows another example where a lateral edge of a recording sheet is used as a reference. In this example, a reference surface 52 is formed on a lateral surface of a frame 53, and there are disposed a tapered roller 61 having a diameter gradually decreasing toward the reference surface 52 and a cylindrical roller 62 urged against the roller 61 and driven by the rotation of the latter. By rotating the roller 61 via a gear 56, a recording sheet 51 is pinched between and fed by the rollers 61, 62. In this apparatus, since the recording sheet is subjected to a feeding force from the roller 61 and is shifted toward the reference surface 52 depending upon an amount of the taper of the roller 61, it is possible to feed the recording sheet while abutting a lateral edge of the sheet against the reference surface 52. In this apparatus, a force acting on the recording sheet 51 to shift it toward the reference surface 52 is determined by the amount of the taper of the roller 61.
However, in the above-mentioned apparatuses, when the leading end P1 of the recording sheet P is used as the reference (FIG. 9), since the registering rollers 92a, 92b having the lengths greater than the length of the leading end P1 of the recording sheet P must be arranged and a space in which the loop is formed in the recording sheet P to correct the skew-feed of the sheet must be established, the structural and spatial losses were greatly increased. Further, since the leading end P1 of the recording sheet P must be temporarily stopped at the nip 93, the time loss was also increased, thus making the speed-up of the feeding of the recording sheet difficult.
On the other hand, when the lateral edge P2 of the recording sheet is used as the reference (FIGS. 10 and 11), although the above drawbacks can be eliminated, there arose a problem that it was very difficult to properly set the skew-feed forces of the skew-feed rollers 97 acting on the recording sheet P. That is to say, if the skew-feed forces were too strong, although the lateral edge P2 of the recording sheet P reached the reference guide 95 for a short time to reduce the skew-feed correcting time, it was feared that the lateral edge P2 of the sheet was more apt to be damaged by the reference guide 95 (refer to FIG. 12). To the contrary, if the skew-feed forces were too weak, although the risk of the damage of the lateral edge P2 of the sheet was reduce, it took a long time to correct the skew-feed of the recording sheet.
Incidentally, if the lateral edge of the recording sheet is damaged, not only the worth of the recording sheet itself is diminished, but also it is more apt to cause the abnormity in the sheet feeding, such as the jamming of the sheet.